Because of the relatively short wear-life of the sprockets in chain and sprocket applications, even when roller chains are used but to a lesser extent compared to non-roller chains, many attempts have been made to improve the sprocket wear-life in numerous ways. One way which has proved very successful is to pivot the teeth on the sprocket and by means of a stationary cam which engages some part of the tooth to gradually move it into contact with a chain link to drive the chain at the entry point of the chain onto the sprocket and then gradually move the tooth away from contact with the chain link at the release point of the chain from the sprocket. In this manner the grinding sliding frictional wear always present when a fixed tooth on a sprocket comes into power transmitting contact with a chain link is completely eliminated. This fixed tooth extreme wear is particularly devasting when the link surface which comes into contact with the tooth is flat because of the characteristics of the particular chain because the sliding friction is thereby greatly increased and the wear-life is thereby greatly reduced.
U.S. Pat. No. 2,931,486 is a typical example of this construction as applied to a corner sprocket and U.S. Pat. No. 1,960,719 is a typical example of this construction as applied to a so-called caterpillar type drive which can be used at any straight section of a chain.
These two patents are believed to represent the closest art to the present invention although U.S. Patent Classification Classes 198--203 and 74--243 include numerous other patents showing this wear reducing feature, any one or more of which may for reasons not now known appeal more to others as being better prior art. Other related classes may also be involved.